fix rare race condition in merkle client rollback (#4809)
fix rare race condition in merkle client rollback (#4809)

- implement @oconnor663's strategy of taking the merkle root before the call is made.
keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client